1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission for a vehicle such as an automobile, and more particularly, to an accumulator control device for controlling a back pressure of an accumulator incorporated in such a hydraulic control device for improving the speed stage shifting performance of the transmission.
2. Description of the Prior Art
It is conventionally practiced to provide an accumulator in a passage for supplying a servo hydraulic pressure to a hydraulically operating friction engaging means such as a clutch or a brake for switching over speed stages of a speed change device such as a planetary gear type speed change device, so as to adjust the servo hydraulic pressure effectively supplied to a servo chamber of the friction engaging means. It is also known to provide a back pressure chamber in the accumulator and to variably control the performance of the accumulator by a control of the hydraulic pressure supplied to the back pressure chamber, as described in, for example, Japanese Patent Laying-open Publication No. 58-152961 and Japanese Patent Laying-open Publication No. 61-149657.
The supply of a hydraulic pressure to the back pressure chamber of the accumulator of the kind described above is controlled by an accumulator control valve. When the accumulator control valve is constructed to be supplied with a signal hydraulic pressure modulated by an electromagnetic valve and to provide a hydraulic pressure for controlling the back pressure of the accumulator according to the signal hydraulic pressure, a fine back pressure control for the accumulator is available by a computer which electrically operates the electromagnetic valve.
However, in such a conventional accumulator control system, if the electromagnetic valve for modulating the signal hydraulic pressure to be supplied to said signal port of the accumulator control valve fails due to a disconnection of a wire, etc., said signal port will be continually supplied with a maximum or a minimum constant pressure, and in accordance with this the hydraulic pressure generated by the accumulator control valve for controlling the back pressure chamber of the accumulator increases to a maximum constant pressure or decreases to a minimum constant pressure, and therefore the back pressure of the accumulator is not controlled according to the output power of the engine, resulting in a serious deterioration of the speed stage shifting performance of the transmission due to an improper operation of the accumulator.